The invention relates to a method for fabricating a photomask and, more particularly, to a method for fabricating a chromeless phase shift mask.
A photomask formed with a pattern is used to realize a desired pattern on a wafer in a process of fabricating a semiconductor device. Since the pattern realized on the photomask is transferred onto the wafer, the process of fabricating the photomask is very important.
As the degree of integration of semiconductor devices increases and pattern sizes are miniaturized, it becomes more difficult to precisely realize the desired pattern on a wafer. Therefore, there has been suggested a phase shift mask on which a phase shift region with a protruded shape is formed, or a chromeless phase shift mask in which a transmissive phase shift groove is formed in a mask substrate and a pattern is formed only by a phase difference of light.
To fabricate a chromeless phase shift mask, a light-shielding layer and a resist layer are formed on a transparent substrate and a photolithography process is performed to form a resist layer pattern that exposes a 180° phase region. The exposed portion of the light-shielding layer is etched using the resist layer pattern as an etch mask to form a light-shielding layer pattern. After the resist layer pattern is removed, the exposed portion of the substrate is etched using the light-shielding layer pattern as an etch mask to form a phase shift region with a groove shape in the substrate.
In the etch process for forming the phase shift region, the transmissive substrate is etched after an etch condition is set to the transmissive substrate. However, the light-shielding layer is exposed to an etch source gas in the process of forming the phase shift region with a groove shape. Therefore, an upper edge portion of the light-shielding layer pattern is attacked, and a critical dimension (CD) and a profile of the light-shielding layer pattern are thus varied to slope a side wall of the light-shielding layer pattern. As such, as the light-shielding layer pattern that functions as an etch mask is varied into the sloped side wall profile, the exposed CD of the bottom of the phase shift region with a groove shape formed in the substrate is narrowed with the etch and the phase shift region has a sloped side wall profile. Also, it is difficult to control an etch depth of the phase shift region and thus obtain an optimal side wall profile. Therefore, technology is needed to solve the problem caused in the process of fabricating the chromeless phase shift mask by controlling the CD and profile of the light-shielding layer.